In a disk file, the most common recording medium is a very flat and smooth aluminum-alloy disk having both of its surfaces coated with a ferromagnetic or ferromagnetic oxide powder dispersed in a resin binder or a ferromagnetic cobalt alloy as a plated or sputter-deposited thin film. Aluminum is a soft metal and therefore, in the attempt to avoid read/write failure caused by the read/write head impacting the disk too forcefully, the aluminum-alloy disk is typically first coated with a hard material before the ferromagnetic cobalt alloy is applied.
In the fabrication of a magnetic-recording disk an aluminum-alloy disk with surfaces that are either diamond tool turned on a lathe or ground by a surface grinder is used. These machined surfaces result in matte finishes. Alternatively, disk substrates could be precision blanked from flat precision cold-rolled aluminum-alloy sheet or other metal strip whose surface finish would replicate that of the work rolls used in the finishing pass of the rolling mill. For example, with work rolls that have been ground and polished to a mirror-bright finish, a metal strip with a mirror-bright surface finish would result. Typically, the aluminum-alloy disk is coated with an electroless-deposited nickel-phosphorus alloy, which is nonmagnetic. However, in order for this plating to adhere properly to the matte surface of an aluminum-alloy disk, a zincate solution is used to dissolve the surface aluminum oxides, hydroxyoxides, and hydrous oxides, and to provide a zinc metal monolayer by replacement reaction. The surface of the electroless-deposited-nickel-phosphorus-alloy-coated aluminum-alloy disk must be lapped and polished prior to the subsequent plating or sputter-deposition of the magnetic layer. Therefore, the fabrication must engage in the laborious lapping and polishing of the disk substrate. This lapping and polishing step is expensive and adds substantial costs to the final disk product. Furthermore, it is extremely difficult to obtain flaw-free electroless-deposited nickel-phosphorus-alloy coatings. Nodules, pits, and bumps occur in these coatings and such defects cause recording errors.
In addition, the electroless-deposited nickel-phosphorus alloy is very prone upon heating to recrystallization, where the nonmagnetic (actually, paramagnetic) nonequilibrium extremely microcrystalline supersaturated-solid-solution single phase of nickel and phosphorus separates into two equilibrium crystalline phases, namely, nickel, which is ferromagnetic, and nickel phosphide. The resulting ferromagnetism renders the media useless for the magnetic-recording application.
The electroless-deposited-nickel-phosphorus-alloy-coated aluminum-alloy disk is also subject upon heating to warpage because of an abrupt coating-substrate interface and the stress concentration at this interface. This warpage also renders the media useless.